This invention relates to a method and apparatus for producing usable energy, and more particularly for producing usable energy from heat sources heretofore inadequate for practical production.
There are many sources of thermal energy that have such low temperatures that they are inadequate for the practical production of power using conventional techniques. These heat sources include direct and indirect geothermal energy, solar energy, industrial waste heat and many others.
The state of the art technologies for converting heat sources into electricity do not perform well at temperatures below about 200xc2x0 Centigrade. This means the vast majority of potential heat sources are not practical energy sources using conventional techniques.
Due to the large amounts of carbon dioxide and other gases in the exhaust of conventional fossil fueled power plants, such power plants have been identified as major contributors to air pollution and the phenomena known as the greenhouse effect within the atmosphere. By successfully utilizing the renewable and non-polluting energy sources such as those mentioned above, the emissions of polluting gases to the atmosphere can be reduced.
Conventional power plants burn coal, natural gas and other fossil fuels to boil water at high pressures. The resulting high-pressure steam is used to drive turbines that are connected to generators. The availability and cost of these fuels make it difficult for most geothermal and impossible for all solar electrical power technologies to compete, given the present state of technology. Large capital costs and the low productivity of renewable energy sources requires renewable energy sources to charge more for electricity than that produced from conventional power plants. An improvement in the efficiency of solar and geothermal power plants and the utilization of heretofore waste heat from industrial processes or operations would be highly desirable.
Thus, the technology of converting low temperature heat sources into usable energy presently lags behind that of conventional power sources when measured by the direct cost of energy output. In addition, the technology of conventional power sources continues to improve, meaning that it is more difficult for renewable energy technologies to become competitive. As in many other situations, the competition is a moving target, improving as time goes on.
A typical geothermal energy driven power plant falls into one of two categories. The first is called direct use in which hot steam from the earth is delivered to turbines which are connected to generators. Typically, the exhaust from the turbines enters a condenser which cools the exhaust vapors to the point where the vapors condense into liquid water. This reduces the exhaust pressure at the outlet and thereby increases the differential pressure across the turbine creating increased horsepower output from the turbine. This allows for smaller turbines to be used for any particular chore. The associated piping and other equipment sizes are lowered respectively, resulting in a lower capital cost for a given power output. However, the heat remaining in gases exiting from the turbine is a considerable fraction of the overall available energy. Consequently, even the most efficient state-of-the-art turbine uses less than half the energy provided by the fuel, the waste heat being delivered to the atmosphere in the form of hot, but relatively low temperature, exhaust gases.
The second basic category of geothermal energy power plant is called a binary process. In this type of process, hot liquids or vapors from the earth are used to heat a second liquid, such as iso-butane, which boils, producing vapors that drive a turbine which is connected to a generator. The exhaust vapors from the turbine are cooled and condensed just as in the direct application described above. Although binary type plants have substantial advantages, a significant proportion of the total energy input is lost in the form of relatively hot water or steam.
Solar energy is used in two general ways to generate heat. The first is a distributed solar collector system in which piping is laid out to absorb energy from the sun. The collector uses transparent plates which allow sunlight to enter the collector, strike the surfaces inside the collector, convert to infrared frequency energy which cannot pass back through the transparent plates. The result is the generation of heat inside the collector which is transferred to the flowing medium inside the pipes, which is typically water. The water becomes hot enough to produce steam when it enters a vessel operating at a lower pressure than the collector piping. The steam is delivered to a turbine driving a generator. Various attempts have been made to promote efficiency, but none are competitive with conventional power plants.
The second type of solar heated power plant is a focused array plant. In this version, mirrors are employed to reflect sunlight on a central receiver. Water passing through piping in the central receiver absorbs the focused sunlight producing high temperatures in the piping. The water boils into high pressure steam delivered to a turbine driving a generator. The array of mirrors tracks the sun via a sophisticated drive system. This type system is expensive.
A third method of converting sunlight into electricity is the photovoltaic method. In this technique, sunlight striking a specially designed material is converted directly into electricity. Large improvements have been made in this technique, but it remains uncompetitive compared to conventional power plants.
The problem is a combination of economics and thermodynamics. If the temperature of the waste water or steam is high enough, there are conventional techniques available to utilize some of the heat economically. Chemical plants and refineries grow like topsy, based in part on the utilization of waste process heat and/or byproducts from older installations. The problem is there is a huge amount of heat at temperatures so low that they cannot be economically used by conventional techniques. On reflection, it will become apparent that every situation where heat is used to generate power results in a quantity of waste heat that cannot be used by current techniques.
Disclosures relevant to the disclosure of this invention are found in U.S. Pat. Nos. 3,955,428; 4,132,077; 4,333,313; 4,729,226; 4,805,410 and 5,040,370.
In this invention, some of the heat from a heretofore uneconomic source may be economically used by directly contacting a liquid from the source with a gas to heat and saturate the gas. This is done in a counterflow contactor, with the liquid typically moving downward and the gas moving upward. The hot gas exits from the contactor and is delivered into the inlet of a motor, typically a turbine which is driven by the heated gas. The turbine typically drives an electrical generator but may be used to drive any other suitable device, depending on the circumstances in which the heat recovery system of this invention is used.
The gas exits from the turbine exhaust into a low pressure condensing vessel where the liquid condenses and separates from the gas. The condensing liquid provides a low pressure discharge for turbine. The gas is delivered into the inlet of a gas compressor and compressed so it will circulate through the contactor. In a preferred embodiment of the invention, the liquid from the condenser may be recirculated through the heat source and thereby use the heat it still contains or the condense liquid may be delivered to a cooling pond or otherwise discarded.
This invention allows the economic use of heat from heretofore impractical sources to be converted into electricity or other usable energy.
It is an object of this invention to provide an improved heat recovery system and process.
A further object of this invention is to provide a heat recovery system and process using a contactor to heat and saturate a gas with a liquid.
Another object of this invention is to provide a heat recovery system and process providing improved efficiencies of operation.
These and other objects and advantages of this invention will become more apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.